Modern vehicles typically have one or more diagnostic systems, generally having separate computer control modules or electronic control units (ECUs) to control various functions of the vehicle. Some examples include a powertrain control module (PCM), engine control module (ECM), transmission control module (TCM), and anti-locking brake system (ABS). The vehicle diagnostic systems often have self-diagnostic capability to detect and alert the driver of problems the vehicle may be encountering. When a problem is found, a diagnostic trouble code or DTC is set within the computer's memory. DTCs are as general or as specific as the manufacturer desires.
To retrieve and decipher DTCs, an auto repair technician needs to connect to the vehicle and be able to communicate with the vehicle. A device, such as one or a combination of a VCI, a diagnostic tool, a scan tool, a PC or the like is typically used to communicate, retrieve, and decipher the DTCs. In one typical implementation, the VCI translates between the vehicle communication protocol and a PC communication protocol. In this case, the communication protocol between the VCI and the PC may be a Universal Serial Bus (USB). In this scenario, the PC serves as the user interface portion of the diagnostic system. Alternatively, one could use a scan tool, which would communicate with the vehicle directly and has its own discrete user interface. The VCI is typically equipped to communicate in various communication protocols such as Controller Area Network (CAN), SAE J1850 VPW, PWM, ISO 9141, and others. These communication protocols may be specific to the various automobile manufacturers and others may be fairly standardized, at least at the physical layer.
A cable is typically used to interface the VCI with the vehicle. Although the VCI typically has a SAE J1962 type connector having 16 pins for various communications, controls and measurements, the use of different pins for different functions varies between the different modules in the vehicle and can also vary with different manufactures of the vehicles. In some instances, the VCI may implement other types of connectors and other types of cables may be substituted for the cable with the SAE J1962 type connector, for example an Ethernet cable or a universal serial bus (USB) cable. Similarly, the VCI may also be connected to other devices and the vehicle itself by one of these cables.
The VCI connection supplies a ground signal to the devices connected to it so that, for example, the diagnostic tool can use that signal as a reference signal. Without a good ground signal, the measurements received by the diagnostic tool may not be accurate. The ground signal can be from the vehicle ground.
A problem known with this approach involves excessive voltage and/or current such as ground loop damage to the associated components. For example, a ground loop can occur in a conductor connecting two points that are intended to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed equipment are a major cause of noise and interference in electrical systems. They can also create an electric shock hazard, since ostensibly “grounded” parts of the equipment, which are often accessible to users, are not at ground potential. When multiple components are connected together, such as the VCI and the diagnostic tool, the components may include ground points which could be at different potentials and cause a ground loop.
Accordingly, it is desirable to provide an apparatus to protect the electrical components of the connected devices from damage, and to protect a user of the devices from injury that could be caused by excessive voltage and/or current such as a ground loop.